Bishop Bernt Eidsvig told domradio.de that after the car bomb in Oslo and the subsequent massacre on the island of Utoya, several hundred thousand people flocked to church.
He said that although there were no Catholics among the victims, many more people attended Mass.
"We Catholics are praying particularly hard for the dead. In Lutheran theology prayers for the dead are not usual so we see this as the part we can play in the mourning," Bishop Eidsvig said.
Breivik, 33, has admitted killing 77 people and injuring 242.
The prosecution is asking for judges to declare him insane when they deliver their verdict on 24 August.